Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, such as described in U.S. Pat. No. 3,455,510 (Rotolico), the heat is provided by a combustion flame and the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. The carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air.
The material alternatively may be fed into a heating zone in the form of a rod or wire such as described in U.S. Pat. No. 3,148,818 (Charlop). In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated.
A newer rocket type of powder spray gun is typified in U.S. Pat. No. 4,416,421 (Browning). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed through an annular opening into the constricted throat of a long nozzle chamber Powder or wire is fed axially within the annular opening into the nozzle chamber to be heated and propelled by the combustion effluent.
Short-nozzle spray devices are disclosed for high velocity spraying in French Patent No. 1,041,056 and U.S. Pat. No. 2,317,173 (Bleakley). Powder is fed axially into a melting chamber within an annular flow of combustion gas. An annular flow of air or non oxidizing gas is injected coaxially outside of the combustion gas flow, along the wall of the chamber. The spray stream with the heated powder issues from the open end of the combustion chamber. A similar device is shown in U.S. Pat. No. 2,544,259 (Duccini et al) wherein the outer flow is taught to be a mixture of oxygen and combustion gas (butane).
Inert spraying to prevent or at least minimize oxidation of metallic or other oxidizable spray material has generally been effected with inert gas plasma spray guns in conjunction with shrouds or chambers to exclude air. U.S. Pat. No. 3,470,347 (Jackson) typifies shrouding of a plasma flame with an inert gas such as argon or helium, and otherwise spraying in open air. U.S. Pat. No. 4,121,083 (Smyth) discloses a flame shroud for a plasma spray gun and, optionally, an annular jet orifice means with cooling water or inert gas to provide an annular curtain effect around the shrouded plasma flame. However, in practice open air spraying with such shrouding of plasma flames requires large quantities of inert gas and reduces oxidation in coatings only to a limited extent.
Plasma spraying in low pressure inert atmosphere chambers, as taught in U.S. Pat. No. 4,328,257 (Muehlberger), can be quite successful in effecting metallic coatings that are dense and free of oxides. However, commercial users of such systems report extensive practical problems and associated costs such as leakage and loading and unloading parts for coating. Dust that adheres to all surfaces in the chamber is potentially flammable, detrimental to pumping equipment, and difficult to clean; down time is typically twice as long as operational time. The plasma spray stream is too wide for small and odd shaped parts used in turbine engines, particularly at the 40 cm spray distance required, and the spray stream is skewed because of the side injection of powder ("target point deviation"). There are reproductibility problems and 84 kw is used to obtain full melting, requiring extensive cooling. The odd shaped configurations for coating require robot-type handling equipment, a major challenge in a chamber.
Therefore objects of the present invention are to provide an improved thermal spray gun and method for spraying low-oxide coatings of oxidizable thermal spray material without the need for an inert atmosphere chamber, to provide an improved method and apparatus for combustion powder thermal spraying at very high velocity, to provide an improved method and apparatus for combustion wire thermal spraying particularly at high velocity, and to provide a method and apparatus for producing dense tenacious thermal sprayed coatings low in oxide at reasonable cost.